feat(ml-backtesting): wire --latency-ns end-to-end + fixture (C12)

The in-flight machinery from C11 is now reachable from the harness +
CLI. New step_decision_with_latency on LobSimCuda:

  latency_ns == 0  → existing immediate-match path (submit_market_immediate
                      kernel fills against current book).
  latency_ns > 0   → host reads market_targets, converts each non-noop
                      into seed_limit_order(active=2, price=very-aggressive,
                      arrival_ts_ns = current + latency_ns). The
                      resting_orders kernel promotes + fills at arrival
                      time, so the order sees whatever book exists then —
                      not the book at decision time.

Aggressive-price heuristic: buy at 1e9 / sell at 0 — the marketability
check (price ≥ best ask for buy, ≤ best bid for sell) unconditionally
crosses at arrival; the kernel then walks book levels for the actual
fill price. This models "market order with latency" correctly because
slippage emerges from the book-walking at arrival, not from a limit
price boundary.

BacktestHarness gains a latency_ns field; harness::run() always calls
step_decision_with_latency now. Also calls sim.step_resting_orders
per event (with trade_signed_vol=0.0 — the Mbp10RawInput trade-flow
hookup is deferred to a trades-feed integration commit) so in-flight
orders get a chance to promote on every snapshot.

bin/fxt-backtest no longer ignores --latency-ns; the flag value
propagates through BacktestHarnessConfig.latency_ns into the kernel
path. Default stays at 100_000_000 (IBKR + Scaleway baseline per
spec §4). Setting --latency-ns 0 selects the legacy immediate path.

Adds latency_in_flight_miss GPU fixture: limit at 5510 submitted
active=2 at T=1s with arrival_ts=T+100ms. step_resting at T+50ms
sees no promotion (still in-flight). Book moves +5 against buyer
during the 100ms window. step_resting at T+150ms promotes the
limit and fills it at the WORSE post-move book (vwap=5505 vs the
original 5500 it would have hit without latency). Slot ends active=0.

11/11 GPU fixtures green on RTX 3050. 33 lib tests still green.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

bin/fxt-backtest/src/main.rs

@@ -55,9 +55,10 @@ struct RunArgs {
     /// Decide every Sth event (matches training stride; default 4).
     #[arg(long, default_value_t = 4)]
     decision_stride: usize,
-    /// Total latency budget in nanoseconds (IBKR+Scaleway baseline = 100ms).
-    /// Not currently consumed by the v1 immediate-match path; reserved for
-    /// follow-up commits that add resting-order in-flight promotion.
+    /// Total submission→fill latency in nanoseconds (IBKR + Scaleway
+    /// baseline = 100_000_000 = 100ms). Propagated to the resting-order
+    /// engine so in-flight orders wait for arrival_ts_ns before crossing.
+    /// 0 = immediate-match path (legacy).
     #[arg(long, default_value_t = 100_000_000)]
     latency_ns: u32,
     /// Target annual volatility in price-unit lot-equivalents
@@ -147,9 +148,9 @@ fn run(args: RunArgs) -> Result<()> {
         annualisation_factor: args.annualisation_factor,
         max_lots: args.max_lots,
         max_events: args.max_events,
+        latency_ns: args.latency_ns,
     };
 
-    let _ = args.latency_ns; // reserved for follow-up resting-limit work
     std::fs::create_dir_all(&args.out)
         .with_context(|| format!("create out dir {}", args.out.display()))?;